This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鶄 combines 青 (blue/green) and 鳥 (bird). The exact historical development is uncertain, but it likely refers to a specific bird with bluish plumage, possibly a heron or similar water bird.
A blue-green (青) bird (鳥) with no clear standalone meaning. Picture a mysterious bluish heron standing in shallow water.
For セイ, imagine this blue-green bird saying its name: 'say' (セイ) as it calls out across the marsh.